For starters, the US population is growing pretty rapidly - probably 60 million more US citizens now compared to 2000. And yeah, baby boomers have their houses, but they haven't yet reached the collective age ranges where they're destined for retirement villages or the afterlife, meaning the big turnover of houses from them to others looking to enter the housing market hasn't happened yet. When that does, I can see the housing shortage quickly turning into a glut.
Also, I think it's got alot to do with people who have the means to move from urban city centers to suburbs much more readily than a generation or two ago, which means more single family homes per capita - US cities are being hollowed out